The invention relates to a method for sorting objects as a function of optical properties of the objects, comprising: conveying the objects over an object plane, all the objects passing a strip of the object plane; detecting light rays originating from the strip by one or more image pick-up units which each have a row of image pick-up elements in an image plane of the image pick-up unit; and for each image pick-up unit: repeated scanning of the row of image pick-up elements to form a primary line signal which contains image information which is a function of the optical properties of the objects and which comprises, for each scan, a primary group of primary intervals corresponding to the image pick-up elements; comparing the line signal with at least one reference signal; and energizing, as a function of the result of comparison, an ejection device, corresponding to the comparison time instant, of a number of ejection devices for the objects arranged downstream of the strip and corresponding to sections of the strip.
A method of this type is known from practice If the method is used for sorting as a function of the light reflection of the objects and the flow of objects in the strip consists of white objects with the exception of one black object, the primary line signal of each image pick-up unit will contain a pulse produced by the detection of the black object. As a consequence of the different arrangements of the image pick-up units, the different optical properties of the image pick-up units and the non-linear relation between the number of image pick-up elements of a row of an image pick-up unit per unit of length of the strip, a pulse of this type occurs, however, at different instants in time and with different widths for the different image pick-up units. Consequently, the information obtained by an image pick-up unit about the optical properties of the strip should be processed independently of information obtained by the other image pick-up units about the optical properties of the strip in order to energize the ejection devices. As a result of this it is not possible to sort over the entire length of the strip with respect to the dimension, extending in the longitudinal direction of the strip, of the objects over which a particular colour, black in the example given, occurs. If more than one image pick-up unit is used, it is furthermore not possible to combine the information obtained by the different image pick-up units about the optical properties of the same or different sections of the objects, detected by the image pick-up units, as a function of the conditions, in accordance with predetermined rules, in order to energize the ejection devices as a function of the combined result. This limits the accuracy of the sorting, undesirable objects not being removed from the flow and too many good objects being ejected.